1. Technical Field
The invention relates to a sensor having a sensor element operating without contact and a housing comprising electronic components as well as an electric connection.
The sensor discussed here represents a sensor operating without contact, with the concretely realized principle being of minor importance. In general it may represent any sensor operating without contact, for example a pressure sensor, an optic sensor, a temperature sensor, a capacitive or inductive sensor, etc. Sensors of the type in question here are used in the most various applications in order to detect physical parameters. In many applications the sensor is subjected to quite considerable environmental influences, such as dirt, dust, water, oil, or other media, which may falsify the measurements or can even destroy the sensor. Excess pressure or vacuum as well as temperature may also lead to an adverse effect or even destruction of the sensor. Accordingly, in such applications it is always necessary to protect the sensor from exterior influences by rendering the housing tightly closed, frequently even hermetically sealed.
2. Description of Related Art
In particular applications it may be necessary to protect the environment from disturbing influences from the sensor. This is particularly the case in a high vacuum or in ultra-clean environments. The environment might be compromised by the emission of contaminating substances from the sensor. For example, when used in an ultra-high vacuum any evaporation of disturbing substances must be prevented because they might destroy the vacuum. In ultra-clean environments, the sensor may not emit any disturbing substances, either. This is the case, for example, in the semiconductor production, in which the environment for the production process must be extremely clean. Particles or gases might interfere with the process by compromising or even destroying reticules, substrates, or optical devices, such as mirrors or lenses. Therefore, under such environmental conditions it is mandatory that no disturbing substances separate from the sensor to the outside.
In order to seal the sensor housing numerous options are known in practice. For example it is possible to use a multi-part housing, with its parts being connected to each other mechanically, for example by a screw connection, clamping, riveting, etc. For sealing purposes, commonly sealing elements made from rubber or plastic are used. Additionally, adhesion or molding methods are known in practice, which are generally used in sensor technology. Here, an adhesion or molding means connects the housing parts to each other and/or to the sensor element. Ultimately the molding material fixates the sensor element inside the housing.
Another option for sealing is given by way of soldering or welding. Here, commonly identical or similar materials, particularly metals, are connected to each other. Additionally, metal-ceramic soldering connections are known from prior art.
The above-stated sealing measures known in practice are disadvantageous such that the sensor element is embodied as a separate component inside the housing, more or less encased by the housing parts. In many types of sensors the measurement is falsified by the housing, in spite of an extremely thin embodiment of the housing wall. For example, in an optic sensor the measurement occurs through a glass window, which for sealing purposes is adhered to the housing. By absorption or diffraction, particularly at the adhesion points, the risk develops that the measurement is influenced. In magnetic sensors the measuring signal is influenced by ferromagnetic materials. In case of inductive or eddy current sensors a metal housing energizes the electromagnetic fields resulting in a reduced measuring signal. For reasons of the required robustness and mechanic stability of the sensor particularly in industrial applications it is usually necessary to produce the housing from metal, regularly from low-cost steel. In order to prevent the housing from excessively influencing the measuring signal the sensor is usually covered and/or sealed at the face with a cap made from plastic, ceramic, or another non-conducting material. The connection of the cap to the metal housing can here occur by way of adhesion, clamping, or using sealing rings, particularly in case of ceramic caps also by way of soldering.
In the sensors of prior art and the methods used for their production it is disadvantageous, though, that the actual sensor element, for example the copper coils of an inductively operating sensor or the condenser plates of a capacitive sensor are further apart and/or separated by the housing from the location of the measurement. This leads to a reduction of sensitivity of the sensor, namely due to the influence of the housing.
In particular in case of distance sensors the measuring distance of the sensor element from the measured object increases due to the housing and/or the material thickness of the housing, because the wall thickness and perhaps existing distances increase the actual measuring distance due to the concrete embodiment of the housing or due to necessary air or adhesion gaps. Additionally, the necessity of a suitable encasing and/or covering via a housing cap leads to additional costs, namely on the one hand material costs and on the other hand costs for assembly.
Sensors with plastic housings are already known in practice, which can be produced more cost-effectively than metal housings. However, plastic housings are disadvantageous in that they absorb moisture/water from the environment, particularly also moisture from the air. This way, capacitive sensors and eddy current sensors are compromised in their function, because due to the accepted water the capacity of the sensor arrangement is changed and regularly a drift of the measurement is caused.